Process of forming oxalate coatings on iron and steel articles



Patented May 23, 1933 UNITED STATES PATENT OFFICE LEO P. CURTIN, OFGRANIBURY, NEW JERSEY, ASSIGNOR TO THE WESTERN UNION TELEGRAPH COMPANY,OF NEW YORK, N. Y., A CORPORATION OF NEW YORK PROCESS OF FORMING OXALATECOATINGS ON IRON AND STEEL ARTICLES No Drawing.

In applications Serial Nos. 539,412 and 591,932, filed May 22, 1931, andFebruary 9, 1932, respectively, in the names of Leo P. Curtin andBernard L. Kline, there are described methods of forming protectivecoatings on iron and steel articles, involving subjecting the cleansurfaces thereof to be coated to the action of solutions containingoxalic acid and ferric oxalate, respectively, as the, essentialingredients.

Thepresent invention relates to a method of forming protective coatingson iron and steel articles by the action of baths or solutions of thetype disclosed in said applications, but is designed to hasten theformation of the coating or shorten the time required for the productionof a given coating. The time element involved in the formation ofprotective coatings on iron and steel articles is of great economicimportance.

In accordance with the present invention the formation of the protectivecoating is hastened by the action of electric current; that is to say,the article is contacted with the coating bath or solution and electriccurrent is passed through the article, serving as the anode, the bath,and an inert cathode such as a carbon rod.

A variety of coating solutions may be employed which are of threegeneral types: (1) solutions of oxalic acid, (2) solutions of ferricoxalate, and (3) solutions containing both ferric oxalate and oxalicacid in varying proportions' The latter type of bath generally givesslightly quicker results and particularly when the ratio of oxalic acidto ferric oxalate in the bath is within the approximate limits of 5parts byilweight of oxalic acid dihydrate, 0 11 0 2 0, to from 1 to 5parts by wei ht of ferric oxalate hexahydrate, F82 C2 4)3.6H2O.

In general t e expedients employed in obtaining coati gs without the useof electric current may be used to advantage also when electric currentis applied in accordance with the present invention. For instance, ifthe surface of the iron or steel article is not clean, e. g. oily orgreasy or rusty, it may be preliminarily cleaned in any suitable Way, asby washing with a solvent for the grease, or by Application filed May 4,1932. Serial No. 609,277.

pickling, or, especially in the case of a rusted surface, the coatingbath may also serve as the cleaning or pickling bath. Oxalic acid isknown to be an efficient pickling agent, but heretofore has not beenused extensively for the purpose on account of its cost.

In the use of oxalic acid alone a 5% solution of the dihydrate of oxalicacid is suit able, although more or lessconcentrated solutions, e. g.from 1 to 10%, may be employed. A small amount of sulfuric acid, e. g.to 1%, or of a ferric salt such as ferric sulfate, e. g. 1% of ferricsulfate may be added to the oxalic acid bath, but this is not essentialand is of little or no utility when applying electric current to hastenthe formation of the coating in accordance with the present invention.The bath may also be heated, e. g. to 95-100 (1, but this expedient alsois of less importance in the present process than when the coating isformed without the aid of electric current. In general the hotter is thebath the more rapid is the formation of the coating, but the bath shouldnot be boiled. Heating and the presence of such additions as sulfuricacid and ferric sulfate serve to hasten the formation of the coating,but the effect thereof is practically negligible when the formation ofthe coating is carried out With the aid of electric current. Thethickness of the coating can, of course, be varied by varying the timeof treatment. Thin coatings generally are formed when the article is tobe painted or lacquered, and serves not only to protect the metalagainst corrosion, but also to bond the paint or lacquer to the article.Thicker coatings are provided when they are to serve as the sole surfacefinish. Such coatings may be oiled to render them impervious.

In the use of solutions of ferric oxalate it is desirable to acidify thesolutions sufficiently with an acid such as sulfuric or oxalic toprevent hydrolysis of.the ferric oxalate and the deposition of insolublematerial, such as ferric hydroxide, which may tend to deposit with orupon the coating on the article and damage it. The quantity and strengthofacid added to the bath should, however, not be suflicient to reactwith the metal of the article and give an objectionable evolution ofhydrogen which tends to disrupt the coating. lVith a 5% solution offerric oxalate a suitable addition of sulfuric acid is 0.5% and ofoxalic acid 1%. The use of oxalic acid as the acidifying acid in aferric oxalate bath gives baths of type (3) referred to above, in which,however, as stated, the preferred composition is 5 parts of oxalic acidto from 1 to 5 parts of ferric oxalate. A suitable bath concentration is20 parts by weight of ferric oxalate and 50 parts by weight of oxalicacid per 1000 parts by weight of solution.

The baths referred to may be restored or replenished from time to timeas required by the addition of suitable reagents such as ferric oxalateor oxalic acid and in case the ferric oxalate becomes reduced to ferrousoxalate it may be reoxidized by the addition of hydrogen peroxide asdescribed in my companion application, Serial No. 608,143, filed April28, 1932. In coating from an acidified ferric oxalate bath the ironcontent of the bath is consumed, while in coating from an oxalic acidbath iron goes into solution, oxalic acid or oxalate radical beingconsumed in both cases, and the restoring of the bath may thereforerequire the addition of iron, e. g. as hydrated ferric oxide, or oxalicacid, or both, or ferric oxalate. The potential and current density arenot at all critical. Potential of from 0.5 to 2 volts have been found tobe quite suitable and current densities of from .005 to .05 amperes persquare centimeter, but it may be noted that when the potential is toolow the efiect becomes negligible, while at high potentials there is atendency to form ferric oxide instead of the complex oxalate of iron onthe surface of the article. A potential of 1.75 volts has been found togive good results. In general the time required to form a coating of agiven thickness by. the use of electric current is less than half thetime required when current is not used. The time required to form acoating depends entirely on the thickness of the coating and it istherefore impossible to state the time required. The thickness of thecoatings has not been measured. It may be said, however, that a thincoating suitable for painting generally is formed in five minutes orless and a. thick coating adapted to serve alone as the finish on thesurface of the article in ten minutes or less. It is noted that thedirection of current flow in the coating operation tends has not beendetermined. They appear to be complex oxalates of iron, which, unlikethe ferrous and ferric oxalates, are insoluble in water and quiteresistant to the solvent action of acetic acid and sodium acetate. Theyare quite firm, relatively non-porous or impervious, and fine grained,strongly adherent and have an attractive olive green color. The coatingsformed by the use of electric current are essentially the same as thosepro-' duced without the use of current.

This application is a continuation-in-part of my application, Serial No.543,748, filed June 11, 1931.

I claim:

1. Method of hastening the formation of oxalate coatings on the surfacesof iron and steel articles exposed to the action of solutions containingsoluble compounds including the oxalate radical which comprisesarranging the article to be coated as the anode in an electrical circuitin a bath of such a solution and im ressing a potential of about from0.5 to 2 v0 ts at a current density of about from .005 to .05 amperesper square centimeter on said article.

2. Method as defined in claim 1 in which the bath is a solution ofoxalic acid.

3. Method as defined in claim 1 in which the bath is a solution offerric oxalate.

4. Method as defined in claim 1 in which the bath is a solutioncontaining both ferric oxalate and oxalic acid.

5. Method as defined in claim 1 in which the bath is a solution ofoxalic acid and ferric oxalate in the proportions of 5' parts by weightof oxalic acid dihydrate to from 1 to 5 parts by weight of ferricoxalate hexahydrate.

In testimony whereof, I afix my signature.

LEO 1P. CURTIN.

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